As mobile equipments such as mobile phones, etc. have recently been used by remarkably increasing numbers of users, their functions and services have extremely improved. Portable wireless communication systems include EGSM (extended global system for mobile communications) and GSM 1800 (global system for mobile communications 1800) widely used mostly in Europe, GSM 1900 (global system for mobile communications 1900) widely used in the U.S., and PDC (personal digital cellular system) used in Japan, etc. From the aspect of convenience by users and efficient use of communications facilities, multi-band mobile phones such as dual-band or triple-band mobile phones, which can utilize pluralities of communication systems, have been developed.
A multi-band mobile phone can utilize pluralities of systems. As a high-frequency part used in such mobile phones, WO 00/55983 discloses a high-frequency switch module for switching transmitting circuits and receiving circuits in different communication systems. This high-frequency switch module comprises first and second filter circuits having different passbands, a switch circuit connected to the first filter circuit for switching a transmitting circuit and a receiving circuit of a communication system A, and a switch circuit connected to the second filter circuit for switching transmitting circuits of communication systems B, C, a receiving circuit of the communication system B and a receiving circuit of the communication system C.
The first and second filter circuits function as circuits for branching transmitting signals and receiving signals of the communication system A and transmitting signals and receiving signals of the communication systems B, C. The switch circuit is a diode switch comprising a diode and a transmission line as main elements, and any one of pluralities of communication systems A, B, C is selected by controlling the diode in an ON or OFF state by applying voltage from control circuits, thereby switching the antenna and the transmitting circuits and receiving circuits of the communication systems A, B, C.
Specific examples of the communication systems A, B, C are, for instance, GSM, DCS 1800 and PCS. GSM corresponds to the above EGSM, DCS 1800 corresponds to the above GSM 1800, and PCS corresponds to the above GSM 1900. Table 1 shows the transmitting and receiving frequencies of each communication system.
TABLE 1CommunicationTransmittingReceiving FrequencySystemFrequency (MHz)(MHz)EGSM880 to 915925 to 960GSM 18001710 to 17851805 to 1880GSM 19001850 to 19101930 to 1990
As shown in Table 1, the transmitting frequencies of GSM 1800 and GSM 1900 are closer to each other than to that of EGSM. Accordingly, GSM 1800 and GSM 1900 can use a common transmitting signal path in a high-frequency circuit. To switch the transmitting signals and receiving signals of GSM 1800 and GSM 1900, a one-input, three-output switch circuit comprising a common port for the transmitting signals of GSM 1800 and GSM 1900, a receiving port of GSM 1800 and a receiving port of GSM 1900 may be used.
As a one-input, three-output switch circuit, WO 00/55983 discloses a diode switch circuit comprising diodes and transmission lines as main elements (FIG. 13). This diode switch circuit comprises cascade-connected, λ/4-switch circuits, to select a transmission mode of GSM 1800/GSM 1900, a receiving mode of GSM 1800 and a receiving mode of GSM 1900 as shown in Table 2 by controlling voltage applied to control terminals VC2, VC3.
TABLE 2ModeVC2VC3GSM 1800 TXV+0(Transmitting)GSM 1900 TXV+0(Transmitting)GSM 1800 RX00(Receiving)GSM 1900 RX0V+(Receiving)
Zero voltage is applied to the control terminals VC2 and VC3 at the time of receiving GSM 1800, to turn off diodes DP1, DP2, DD1 and DD2. With the diode DD1 in an OFF state, there is large impedance between a connecting point 1P2 and the transmitting circuit TX1, TX2 of GSM 1800/GSM 1900. With the diode DP1 in an OFF state, there is large impedance between a connecting point IP3 and the receiving circuit RX2 of GSM 1900. Accordingly, the connecting point IP2 is connected to the receiving circuit RX1 of GSM 1800 via two transmission lines ld3 and lp2
Usually, the transmission line ld3 has such length that its resonance frequency is within a frequency range (1710–1910 MHz) of transmitting signals of GSM 1800 and GSM 1900, and the transmission lines lp2 has such length that its resonance frequency is within a frequency range (1930–1990 MHz) of a receiving signal of GSM 1900, with their characteristic impedance designed to be 50Ω.
However, intensive research has revealed that when the switch circuit is formed by strip lines, etc. in a multi-layer substrate, the reduction of size, particularly thickness, of the multi-layer substrate provides a receiving signal output port RX1 of GSM 1800 and a receiving signal output port RX2 of GSM 1900 with impedance of about 70–80Ω, larger than 50Ω, by parasitic capacitance, etc., even though the characteristic impedance of the transmission lines ld3 and lp2 is set at 50Ω, resulting in large loss in receiving signals from the output ports RX1, RX2. WO 00/55983 does not recognize this problem, much less provides any solutions.